Material eroded from ancient mountains is carried by rivers into depressions filled with seawater, where it settles as sediment. At that time, Luxembourg was part of one such depression, the Paris Basin.
Under high pressure and at temperatures of several hundred degrees, the sediments are transformed—depending on their composition—into rock of varying degrees of hardness (e.g., limestone, sandstone …). Because the layers alternate between harder and softer, a landscape of terraced layers eventually forms on the surface, as seen in the Gutland region.
Iron also precipitates in these layers in the form of iron oxide (Fe₂O₃) along with other materials. The Luxembourg-Lorraine deposit formed a good 150 million years ago and covers an area of 106,000 hectares. Of this, 3,670 hectares are in Luxembourg and 350 hectares are in Belgium.
The thickness of the ore deposits ranges from 10 to 60 m. The average iron content is about 28%. In some places, up to 10 distinct layers can be identified, which differ in their composition, iron content, and, in some cases, even their color. Because of its low iron content, the iron ore is called“Minette”(a diminutive of “mine”).
The strata have a dip of 1–2% toward the southwest. In southern Luxembourg, they outcrop at the surface, making open-pit mining possible there and allowing miners to drive nearly horizontal tunnels into the iron ore strata for underground mining. In France, where the ore layers were deeper, they could often only be reached via shafts.
Oolitic iron ore
Minette, which is found in Luxembourg and Lorraine, is an oolitic ore. Iron oxides accumulate around a core, forming ore grains with a concentric-layered structure (oolites) up to 0.3 mm in diameter.
These oolites are held together by a calcareous or siliceous cement.
Iron content of Luxembourg’s ores:
calcareous ore: 22–35%
siliceous ore: 25–37%
Since the local ores contain 1.7% phosphorus, the pig iron produced from them could only be processed into high-quality steel using the Thomas process. A byproduct of this process was Thomas slag, which, when finely ground into Thomas meal, was used in agriculture as a mineral fertilizer.
Mining
Mining can be carried out via open-pit mining or underground mining. In Luxembourg, adits were driven that followed the natural dip of the ore deposits (1–2%). On the French side, where the deposits lie deeper, shafts with headframes often had to be constructed. Underground, the pillar-retreat method (dépilage) was used.
Major open-pit mines included, for example, Schlammfeld near Petingen, Lallingerberg near Esch, and Hutberg near Rümelingen.
Concessions
The landowner had the right to mine the iron ore beneath his property only if it was not too deep. If the ore deposit was covered by a layer of over 23 m (only 6 m east of the Alzette), a permit from the state—which was subject to a fee—was required to mine the ore (concession).
The Iron Ore Layers
The various layers are identified by color. Not all layers are found everywhere or in every mine. Sometimes the mineralogical composition of a layer also varies from one region to another.